Technology is known for measuring traffic volume based on a captured image from a monitoring camera, for example, as described in Japanese Patent Application Laid-Open (JP-A) No. 2011-198030 (Patent Document 1). In technology hitherto, calibration is first performed for a captured image. A road region is specified in the captured image by such calibration. Then a road parameter is computed to convert coordinates in the captured image to coordinates in real space. Then in technology hitherto, a vehicle lane region is detected in the captured image, and a processing region is set based on the vehicle lane region. The processing region is a region observed for the passing of vehicles. Namely, a vehicle is detected when a vehicle is superimposed on the processing region. The processing time is shortened and the measurement precision raised by setting the processing region.